This invention relates to a device for the injection of fuel into the combustion chamber of a gas turbine having at least one swirler arranged in an air path with at least one swirler vane and with at least one fuel injection nozzle.
According to the state-of-the-art, the fuel must be mixed with the compressed air to enable subsequent combustion in the combustion chamber of a gas turbine. For this purpose, at least one, two or three air paths and at least one fuel path are provided in the burner. A swirler is frequently arranged in the air path and it can be of the axial, diagonal or radial type. In the burner, the air and the fuel are mixed appropriately. In the process, the fuel is either introduced in the form of a thin lamella or a lamella-type stream between two possibly swirled air streams or transported by an air stream to an atomiser edge as a thin film. Here, the fuel lamella or the fuel film are atomised and mixed with the air.
In an alternative approach, several radial, diagonal or axial injection holes are provided through which the fuel is introduced into the air stream from a central or annular body or from a swirler vane.
A design of the said type is shown in Specification DE 195 32 264 A1, for example.
As long as the swirlers and the respective geometrical dimensions are small, the fuel jets will mix well with the air. Here, a stoichiometrically lean mixture in the flame allows pollution emission, e.g. nitrogen oxides, to be reduced. To obtain this lean mixture in the primary zone, the flow cross-section of the burners was increased and the amount of dilution air from the dilution-air ports of the flame tube was constantly reduced in the past.
It was found, however, that a further increase of the flow cross-section is not accompanied by a corresponding reduction of the pollution emission, for example the nitrogen oxide emission, if a certain flow area of the burner is exceeded. This is due to the fact that the mixing process of fuel and combustion air will depreciate the more the flow cross-section of the burner is increased. If the burner flow areas are very large, a major portion of the air will no longer be involved in the mixing process with the fuel.
With the state-of-the-art pressure ratios currently employed for gas turbines, the penetration depth of the fuel jets is very limited; it amounts to a few millimetres only, for example to 6 mm. Accordingly, the size of the usable flow area of the burner is very much confined if fuel injection is accomplished from a central or annular body. Injection of fuel from a swirler vane greatly increases the surface of the fuel-guiding components. Consequently, these must be actively cooled to cater for the currently employed compressor exit temperatures. This increases the engineering effort as well as the costs and the failure probability.